Yoon Suk Yeol's promise to be a different leader in South Korea ...

South Korea's president Yoon Suk Yeol has been impeached in a historic vote, marking the end of a tremulous week since his desperate attempt to impose martial law spectacularly failed.

It's a tremendous fall from grace for Mr Yoon, who was elected to the top job after building a successful career as a public prosecutor and crusader against corruption.

In his final week of power, the president's tone changed from apologetic to the Korean people to one of defiance, stubbornness and aggression.

He reversed a previous agreement that he would leave his post early next year to allow for a smooth transition of power.

Mr Yoon became increasingly isolated, touting conspiracy theories about stolen elections, reportedly relied on far-right YouTube channels for information and perhaps even sought advice from a fortune teller.

Members of his own party, having abstained from the first impeachment vote, switched sides.

In the end, 12 supported the impeachment — four more than the eight required.

Huge crowds outside the National Assembly erupted into cheers when the vote was confirmed.

"I'm honoured to be part of this historical moment," said protester Cho Seo Yeon.

"When there was the announcement the impeachment passed, I cried hard because I was so touched."

Mr Yoon said in a televised address he would step down after immediately losing his presidential powers, but also vowed to continue the fight.

Even though the Constitutional Court may overturn the impeachment, his reputation is in tatters.

Protesters calling for Mr Yoon's impeachment gathered outside Seoul's National Assembly ahead of the vote. (AP: Lee Jin-man)

Mr Yoon became a national celebrity when he brought down Park Geun-hye, the nation's first female president, over corruption charges.

Ms Park was considered a trailblazer in male-dominated South Korea, but was undone when it emerged her longtime political advisor and friend Choi Soon-sil used her access to give some of South Korea's largest corporations an unfair advantage.

After her impeachment, former left-leaning president Moon Jae-in picked Yoon Suk Yeol to prosecute her as Mr Yoon had earned the reputation as a man of principles and impartiality.

Park Guen-hye was considered a trailblazer in male-dominated South Korea. (Reuters)

"The Moon administration saw him as someone who would work with conviction and without political bias," said veteran journalist turned columnist Byun Sang Wook.

"Park Geun-hye went to prison [and] Yoon emerged as a hero who took down the mighty former President Park Geun-hye and those forces by rooting out corruption."

But the honeymoon period with the Moon administration was short-lived, with Mr Yoon deciding to go after after the then-justice minister — an ally of Moon Jae-in.

The conservative People Power Party, having spent 10 years in opposition, saw an opportunity.

"Yoon's battle with the Moon Administration gave him the profile to run for president," said Joel Atkinson, an expert in Korean politics at the Graduate School of International and Area Studies.

"South Korea has a large number of non-committed voters in the centre, and his image as a non-ideological principled prosecutor of corruption was to his advantage."

In the lead-up to the 2022 presidential election, Mr Yoon promised policies that appealed to both the left and right wing, depriving the opposing Democratic Party the ability to shine.

He also tapped into growing resentment among young men that the pivot towards female empowerment was unfair.

It was the sign of his populist politics.

Yoon Suk Yeol at a 2022 campaign rally in Seoul. (Reuters: Kim Hong-Ji)

"There was nothing he said he wouldn't do," Mr Byun said.

"He said he would do everything well."

But securing what he wanted proved much harder than he led voters to believe.

The Democratic Party controlled the parliament and was able to stifle key policies, such as abolishing the Minister of Gender Equality.

President Yoon pushed for policies that favoured big business, such as deregulation and tax cuts, which he touted would stimulate the economy and bolster living standards.

"Yoon made statements supporting classical liberalism, market economies, and the defence of South Korea's institutions against anti-liberal democratic threats — hints of his future political trajectory," Mr Atkinson said.

But when his policies failed to ease cost-of-living pressures, voters increasingly viewed the president as out of touch.

Just before the mid-term parliamentary elections, Mr Yoon demonstrated he did not understand the cost of a spring onion, a staple in South Korean cooking.

It was the final straw after a long slide in the approval ratings.

The Democratic Party won the mid-terms in a landslide, further strengthening its power and ability to block his agenda.

Despite pledging to clean up politics, Mr Yoon's administration has been bogged down by its own corruption scandals.

Mr Yoon's wife, Kim Keon Hee, has come under controversy for a raft of allegations. (AP: Kim Hong-Ji)

Arguably the most famous involves his wife, Kim Keon Hee, who was filmed accepting a $3,400 luxury Christian Dior handbag from a pastor.

She had already come under controversy for a raft of allegations, such as failing to pay taxes and taking kickbacks for hosting art exhibitions.

Mr Yoon has long argued South Korea had been seized by the left and lashed out at the Democratic Party, academia and the media.

When he declared martial law last week, he cited the need to "safeguard a liberal South Korea from the threats posed by North Korea's communist forces".

It's widely accepted this was a jab at the Democratic Party, as the hawkish Mr Yoon has long believed the opposition is too soft on Pyongyang, and the martial law was a desperate attempt to break a political deadlock.

But another theory gaining traction is it was a desperate attempt to shield his wife from prosecution.

"Whether his wife encouraged [martial law], or how his wife gave advice, is among the biggest suspicions now," said Byun Sang Wook.

"Nothing has been revealed concretely yet."

In his last moment of defiance, Mr Yoon gave a televised address where he alluded to voter fraud and demanded an investigation.

The electoral commission has rejected any notion the system has been compromised and, for now at least, the allegations have failed to gain mainstream traction.

"The bottom line is not where he got the information [to declare martial law], but that the decision was profoundly wrong-headed, doing great damage to South Korean democracy, but also to his own party and himself," Joel Atkinson said.

"It wouldn't have gotten to this dire point if he had figured out that he needed to stop acting like a prosecutor and start acting like a politician.

"The result was a frightening farce."